Plasma fluorination treatment of porous materials

ABSTRACT

The application discloses articles and methods of plasma fluorination treatment that employ a capacitively-coupled system to fluorinate porous articles. The methods include placing the article to be treated within an ion sheath adjacent to an electrode and placing the article to be treated between powered and grounded electrodes separated by about 25 mm or less.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Patent ApplicationNo. 60/341,564, Filed Dec. 14, 2001, which is incorporated by reference.

BACKGROUND

Plasma-deposited fluorocarbon coatings can impart desirable properties,such as low surface energy, water-repellency, soil resistance, anddurability, to a treated article. A charge can be imparted to thetreated article, which makes the article suitable for use in items suchas aerosol filters, face masks, air filters, and electrostatic elementsin electro-acoustical devices such as microphones, headphones, andelectrostatic recorders. Accordingly, plasma fluorination methods thatcan quickly and efficiently produce an article with a fluorocarboncoating are desired.

SUMMARY OF INVENTION

One aspect of the present invention features a plasma fluorinationmethod to fluorinate porous articles, both on the surface and in theinterior. It also features the resulting articles.

One aspect of the present invention is a method of fluorinating a porousarticle comprising: providing a reaction chamber having acapacitively-coupled system comprising at least one grounded electrodeand at least one electrode powered by an RF source; generating afluorine-containing plasma in the chamber thereby causing an ion sheathto form adjacent to the electrodes; placing a porous article in the ionsheath of the powered electrode; and allowing reactive species from theplasma to react with the article surface and interior whereby thearticle becomes fluorinated.

Another aspect of the present invention is a method of fluorinating aporous article comprising: providing a reaction chamber having acapacitively-coupled system comprising at least one electrode powered byan RF source and at least one grounded electrode that is substantiallyparallel to the surface of the powered electrode and separated from thegrounded electrode by about 25 millimeters or less; generating afluorine-containing plasma in the chamber at a pressure of about 40Pascal or less; placing a porous article between the substantiallyparallel electrodes and outside of the ion sheath; and allowing reactivespecies from the plasma to react with the article surface and interiorfor a total treatment time of over two minutes whereby the articlebecomes fluorinated.

Another aspect of the present invention is a method of fluorinating aporous article comprising: providing a reaction chamber having acapacitively-coupled system comprising at least one electrode powered byan RF source and at least one grounded electrode that is substantiallyparallel to the surface of the powered electrode and separated from thegrounded electrode by about 25 millimeters or less; generating afluorine-containing plasma in the chamber thereby causing an ion sheathto form adjacent to the electrodes; placing a porous article in the ionsheath of the grounded electrode; and allowing reactive species from theplasma to react with the article surface and interior for a totaltreatment time of about 30 seconds to about 5 minutes whereby thearticle becomes fluorinated.

Another aspect of the present invention is a method of fluorinating aporous article comprising: providing a reaction chamber having acapacitively-coupled system comprising at least one electrode powered byan RF source and at least one grounded electrode that is substantiallyparallel to the surface of the powered electrode and separated from thegrounded electrode by about 13 millimeters or less; generating afluorine-containing plasma in the chamber thereby causing an ion sheathto form adjacent to the electrodes; placing a porous article between theelectrodes; and allowing reactive species from the plasma to react withthe article surface and interior whereby the article becomesfluorinated.

The methods may include embodiments wherein the process is continuousand/or wherein the treatment time is less than about 60 seconds.

The porous article to be treated may be selected from the groupconsisting of foams, woven materials, nonwoven materials, membranes,frits, porous fibers, textiles, and microporous articles. The articlemay have pores smaller that the mean free path of any species in theplasma. The article may have two parallel major surfaces and may betreated on one or both major surface.

The methods may be carried out with the electrodes separated by about 25millimeters or less. In some embodiments, the electrodes are separatedboy about 16 millimeters(mm) or about 13 mm. Another aspect of theinvention is an article comprising at least one fluorinated porous layerhaving a basis weight of about 10 to about 300 gsm and a thickness ofabout 0.20 to about 20 mm, wherein the layer has a Q₂₀₀ of greater thanabout 1.1. The layer may have an effective fiber diameter of about 1 toabout 50 μm.

Another aspect of the invention is an article comprising a compositelayer comprising a non-fluorine containing porous layer and a plasmafluorinated layer affixed to the surface and interior of the porouslayer, wherein the composite layer has at least 3700 ppm fluorine or, inanother embodiment, at least 5000 ppm.

Another aspect of the invention is an apparatus for fluorinating asubstrate comprising a vacuum chamber, a capacitively-coupled systemwithin the chamber comprising at least one electrode powered by an RFsource and at least one grounded electrode substantially parallel to thepowered electrode wherein the electrodes are separated by about 25 mm orless, e.g., about 16 mm or 13 mm, and a means for generating afluorine-containing plasma throughout the entire chamber.

The powered electrode may comprise one or more rotating drums. Theapparatus can comprise an asymmetric parallel plate reactor.

As used in this invention:

“microporous membrane” means a membrane having pore sizes with a lowerlimit of about 0.05 μm and an upper limit of about 1.5 μm;

“plasma fluorocarbon” means a material deposited from a plasmacomprising fluorocarbon species;

“plasma fluorination” means thin film deposition, surface modification,and any other plasma-induced chemical or physical reaction that canfluorinate an article;

“porous article” means an article having pathways open to at least onesurface;

“Q₂₀₀” means the quality factor rating of a filter; the procedure fordetermining Q₂₀₀ is set forth in the Examples section of thisapplication, and

“substantially parallel” means the electrodes are substantially the samedistance from each other along their entire lengths, includingconcentric electrodes.

An advantage of at least one embodiment of the present invention is thatit provides a continuous plasma fluorination method, which allows forefficient, i.e., faster, processing of articles, especially continuousarticles, e.g., long sheets of material, as are used in roll-to-rollprocessing.

Another advantage of at least one embodiment of the invention is that itprovides a durable fluorination treatment through the bulk of porousarticles, including microporous membranes.

Another advantage of at least one embodiment of the present invention isthat treatment efficiencies can be obtained by placing the article to betreated within an ion sheath.

Another advantage of at least one embodiment of the present invention isthat fluorination efficiency may be achieved by reducing the spacebetween the powered and grounded electrode to about 25 mm or less.

Other features and advantages of the invention will be apparent from thefollowing drawings, detailed description, and claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 depicts a parallel plate plasma apparatus for performing theplasma fluorination of the present invention.

FIG. 2 depicts a single drum plasma apparatus for performing the plasmafluorination of the present invention.

DETAILED DESCRIPTION

The present invention provides a plasma fluorination method tofluorinate a porous article.

One method embodiment involves providing a reaction chamber having acapacitively-coupled electrode system wherein an ion sheath is formedadjacent to at least one electrode when a plasma is generated in thesystem. The ion sheath is an area adjacent to an electrode in which ionbombardment is prevalent. The porous article to be treated is placedwithin the ion sheath.

This method of the invention can be especially effective for articleswith small pores because the ion sheath can force chemical species fromthe plasma into small pores of the articles being treated. This resultsin surprisingly fast fluorination of the pore interiors. It was notexpected that plasma fluorination could be achieved within small pores,especially in cases where the pores are smaller than the mean free pathof any species in the plasma.

The mean free path (MFP) for a particular species is the averagedistance traveled by a species before it collides with another species.The MFP depends in part on pressure because the proximity of speciesinfluences the collision frequency. For example, at 0.13 Pa (1 mTorr)and room temperature, the mean free path of an argon atom is 80 mm. SeeBrian Chapman, Glow Discharge Processes, 153 (John Wiley & Sons, NewYork 1980). Most other gases, including those used in the presentinvention, are within three times (i.e. 26-240 mm) of this value at thispressure. In the range of pressures useful for plasma fluorination, themean free path of argon varies from 80 mm to 0.08 mm (or 80 microns).Other gases would have similar variations.

In plasma-treating a porous article, if the pore size is smaller thanthe mean free path of the species in the plasma (i.e., smaller thanabout 20 microns), normally the free radical species generated in theplasma will collide with the pore walls near the pore opening. The freeradicals will react with molecules in the pore walls near the poreopening rather than traveling into the depths of the pores. Therefore,one would not expect the plasma fluorination to penetrate into thedepths of the pores, especially when the pores have tortuous paths.

Another method embodiment of the present invention involves providing areaction chamber having a capacitively-coupled electrode system in whicha powered and grounded electrode are spaced about 25 mm (one inch), orless, apart and the porous article to be treated is suspended betweenthe two electrodes and outside an ion sheath. In this embodiment, thechamber pressure is maintained at about 40 Pa or less and the totaltreatment time is over 2 minutes. This treatment method results intreated articles having a higher fluorine content, and better oilrepellency, than similar articles treated in a system in which thegrounded and powered electrode are farther apart and the chamberpressure is higher than about 40 Pa.

Another method embodiment of the present invention involves providing areaction chamber having a capacitively-coupled system comprising atleast one electrode powered by an RF source and at least one groundedelectrode that is substantially parallel to the surface of the poweredelectrode and separated from the grounded electrode by about 25millimeters or less; generating a fluorine-containing plasma in thechamber thereby causing an ion sheath to form adjacent to theelectrodes; placing a porous article in the ion sheath of the groundedelectrode; and allowing reactive species from the plasma to react withthe article surface and interior for a total treatment time of about 30seconds to about 5 minutes whereby the article becomes fluorinated.

Another method embodiment of the present invention involves providing areaction chamber having a capacitively-coupled system comprising atleast one electrode powered by an RF source and at least one groundedelectrode that is substantially parallel to the surface of the poweredelectrode and separated from the grounded electrode by about 13millimeters or less; generating a fluorine-containing plasma in thechamber thereby causing an ion sheath to form adjacent to theelectrodes; placing a porous article between the electrodes; andallowing reactive species from the plasma to react with the articlesurface and interior whereby the article becomes fluorinated.

Porous Articles

Porous articles suitable for use in the present invention include foams,nonwoven materials, woven materials, membranes, frits, porous fibers,textiles, and microporous articles. These articles may have pore sizesof about 0.05 micrometers or greater.

The porous articles may be made from, e.g., polymers, metals, glasses,and ceramics. Suitable polymers for the above articles includepolyolefins such as, e.g., polypropylene, polyethylene,poly-(4-methyl-1-pentene), and combinations thereof, halogenatedvinylpolymers (e.g., polyvinyl chloride), polystyrene, polycarbonates,polyesters, polyamides, and combinations thereof. The nonwovens can beformed by a variety of methods, including but not limited to, carding,use of a rando-webber, spunbonding, hydrolacing, or blown microfibers.The textiles and cloths can be formed as nonwovens or as knit or wovenmaterials. The textiles and cloths preferably have a basis weight in therange of about 10 to 500 grams per square meter more preferably about 15to 300 grams per square meter. Porous frits synthesized from polymers,metals, glasses and ceramics are available commercially in various poresizes. The pore size typically varies between 1 and 250 microns and thefrits may have a void volume of between 20 and 80%. Typical applicationsof frits include filtration, support media for membrane cartridges,solvent filters, diffusers, fluidization supports, bio-barriers, nibsfor writing instruments, chromatographic support media, catalysissupport media, etc. Porous fibers are also commercially available.Typical diameters for these fibers are up to and around 100 μm andtypical pore sizes are from about 0.001 μm (10 Å) to about 10 μm (1000Å).

Suitable microporous films may be prepared by thermally-induced phaseseparation (TIPS) methods such as those described in U.S. Pat. No.4,539,256 (Shipman), U.S. Pat. Nos. 4,726,989; 5,120,594 (Mrozinski);and U.S. Pat. No. 5,260,360 (Mrozinski et al.) which describe such filmscontaining a multiplicity of spaced, randomly dispersed, equiaxed,nonuniform shaped particles of a thermoplastic polymer. These filmstypically have pore sizes with a lower limit of about 0.05 micrometersand an upper limit of about 1.5 micrometers.

A suitable porous material may have a basis weight of 10 to 300 gsm(grams per square meter) and a thickness of 0.20 to 20 mm. The porousmaterial also may have an effective fiber diameter of 1 to 50 μm.

The porous articles can be any shape, e.g., sheets, rods, cylinders,etc., as long as they can be placed within an ion sheath that surroundsan electrode. Typically the articles will be sheet-like with two majorparallel surfaces. The articles may be discrete articles or may becontinuous sheets of material. They may have any level of hydrophobicityor hydrophilicity before they are treated.

The resulting fluorinated porous article may be used alone or may beincorporated into another article. For example, it may be incorporatedinto a multi-layer (two or more layers) article in which the otherlayer(s) are fluorinated or unfluorinated and are porous or nonporous.The multi-layer article may be made by any method known in the art,e.g., lamination, physical bonding, etc.

Porous filter media are frequently employed to filter air containingsolid and/or liquid particles. The particles removed are often toxic ornoxious substances. Scientists and engineers have long sought to improvefiltration performance of air filters. Some of the most effective airfilters use electret articles. Electrets are dielectric articles thatexhibit a lasting charge, that is, a charge that is at leastquasi-permanent. The term “quasi-permanent” means that the timeconstants characteristic for the decay of the charge are much longerthan the time period over which the electret is used.

The charged nature of the electret enhances the filter's ability toattract and retain particles such as dust, dirt, and fibers that arepresent in the air. Electrets have been found to be useful in a varietyof applications including air, furnace and respiratory filters, facemasks, and electro-acoustic devices, such as microphones, headphones,and electrostatic recorders.

Over the years, various methods of making and improving the filtrationperformance of nonwoven fibrous electrets have been developed. Thesemethods include, e.g., bombarding fibers with electrically chargedparticles as the fibers issue from a die orifice, corona charging anonwoven fibrous web, and hydrocharging a nonwoven fibrous web.

While performance is enhanced through the use of electret charged media,degradation in filter efficiency during exposure or loading of aerosolscontaining an oily mist has been exhibited in some media. This change inperformance during loading prompted the National Institute forOccupational Safety and Health (NIOSH) to specify testing that requiresrespirators used in oily mist environments to be exposed to 200 mg ofdioctyl phthalate (DOP) during certification testing. In order todetermine the benefits of the filters of this invention, the filterpenetration was measured after exposing the sample to 200 mg ofaerosolized DOP.

In addition to penetration, pressure drop of the filter is a keymeasurement in designing a filter. Pressure drop is defined as areduction in static pressure within an air stream between the upstreamand downstream sides of a filter through which the air stream passes. Alower pressure drop allows air to flow through the medium more easily.Lower pressure drop is typically preferred because it allows less effortor energy to be used to achieve the desired flow. This is true whetherthe filter is employed as a respirator, which a user breathes through; abattery powered air-purifying respirator; or a home furnace filter.

To ease the comparison and the design of filters, researchers oftencombine penetration and pressure drop into a single term of QualityFactor, i.e., the quality of the filtration performance of the material.In this application, quality factor is based on penetration and pressuredrop after exposure to of 200 mg of dioctyl phthalate, as explained inmore detail in the Examples section. The Quality Factor Rating isreferred to as Q₂₀₀.

Some articles of the present invention have Q₂₀₀ ratings over 1.1, andin some cases, as high as 1.53. Some articles also have fluorineconcentrations of over 3700 ppm, and in some cases, as high as 5000 ppmor more.

Apparatus

An apparatus suitable for the present invention provides a reactionchamber having a capacitively-coupled system with at least one electrodepowered by an RF source and at least one grounded electrode. In someembodiments, a grounded electrode is separated from the poweredelectrode by about 25 mm or less.

A suitable reaction chamber is evacuable, has means for generating afluorinated plasma throughout the entire chamber and is capable ofmaintaining conditions that produce plasma fluorination. That is, thechamber provides an environment which allows for the control of, amongother things, pressure, the flow of various inert and reactive gases,voltage supplied to the powered electrode, strength of the electricfield across the ion sheath, formation of a plasma containing reactivespecies, intensity of ion bombardment, and rate of deposition of a filmfrom the reactive species. Aluminum is a preferred chamber materialbecause it has a low sputter yield, which means that very littlecontamination occurs from the chamber surfaces. However, other suitablematerials, such as graphite, copper, glass or stainless steel, may beused.

The electrode system may be symmetric or asymmetric. Preferred electrodesurface area ratios between grounded and powered electrodes for anasymmetric system are from 2:1 to 4:1, and more preferably from 3:1 to4:1. The ion sheath on the smaller powered electrode will increase asthe ratio increases, but beyond a ratio of 4:1 little additional benefitis achieved. Placing the sample on the powered electrode is generallypreferred because DC bias would not be shunted to ground. Bothelectrodes may be cooled, e.g., by water.

Plasma, created from the gas within the chamber, is generated andsustained by supplying power (for example, from an RF generatoroperating at a frequency in the range of 0.001 to 100 MHz) to at leastone electrode. The RF power source provides power at a typical frequencyin the range of 0.01 to 50 MHz, preferably 13.56 MHz or any whole number(e.g., 1, 2, or 3) multiple thereof. The RF power source can be an RFgenerator such as a 13.56 MHz oscillator. To obtain efficient powercoupling (i.e., wherein the reflected power is a small fraction of theincident power), the power source may be connected to the electrode viaa network that acts to match the impedance of the power supply with thatof the transmission line (which is usually 50 ohms resistive) so as toeffectively transmit RF power through a coaxial transmission line. Adescription of such networks can be found in Brian Chapman, GlowDischarge Processes, 153 (John Wiley & Sons, New York 1980). One type ofmatching network, which includes two variable capacitors and aninductor, is available as Model # AMN 3000 from RF Power Products,Kresson, N.J. Traditional methods of power coupling involve the use of ablocking capacitor in the impedance matching network between the poweredelectrode and the power supply. This blocking capacitor prevents the DCbias voltage from being shunted out to the rest of the electricalcircuitry. On the contrary, the DC bias voltage is shunted out to thegrounded electrode. While the acceptable frequency range from the RFpower source may be high enough to form a large negative DC self bias onthe smaller electrode, it should not be so high that it creates standingwaves in the resulting plasma, which is inefficient for plasmafluorination.

The articles to be treated may be placed in, or passed through, theevacuable chamber. In some embodiments, a multiplicity of articles maybe simultaneously exposed to the plasma during the process of thisinvention.

In an embodiment in which the article is treated within an ion sheath,plasma fluorination of discrete planar articles can be achieved, forexample, by placing the articles in direct contact with the poweredelectrode. This allows the article to act as an electrode due tocapacitive coupling between the powered electrode and the article. Thisis described in M. M. David, et al., Plasma Deposition and Etching ofDiamond-Like Carbon Films, AIChE Journal, vol. 37, No. 3, p. 367 (1991).In the case of an elongated article, the article may optionally bepulled through the vacuum chamber continuously, while maintainingcontact with an electrode. The result is a continuous plasmafluorination of the elongated article.

FIG. 1 illustrates a parallel plate apparatus 10 suitable for thepresent invention, showing a grounded chamber 12 from which air isremoved by a pumping stack (not shown). Gases to form the plasma areinjected radially inward through the reactor wall to an exit pumpingport in the center of the chamber. Article 14 is positioned proximateRF-powered electrode 16. Electrode 16 is insulated from chamber 12 byTeflon support 18.

It is not necessary to confine the plasma between the electrodes. Theplasma may fill the entire chamber without diminishing the effectivenessof the plasma fluorination. However, the plasma will usually appearbrighter between the two electrodes.

FIG. 2 illustrates single-drum apparatus 100 that is also suitable forthe present invention, especially the method embodiment that employs anion sheath. This apparatus is described in more detail in U.S. Pat. No.5,948,166. The primary components of apparatus 100 are rotating drumelectrode 102 that can be powered by a radio frequency (RF) powersource, grounded chamber 104 that acts as a grounded electrode, feedreel 106 that continuously supplies article 108, which is to be treated,and a take-up reel 110, which collects the treated article. A concentricgrounded electrode (not shown) can be added near the powered electrodeso spacing can be controlled.

Article 108 is a long sheet that, in operation, travels from feed reel106, around drum electrode 102 and on to take-up reel 110. Reels 106 and110 are optionally enclosed within chamber 104, or can be outsidechamber 104 as long as a low-pressure plasma can be maintained withinthe chamber.

The curvature of the drum provides intimate contact between the articleand the electrode, which ensures that the article remains within the ionsheath, irrespective of other operating conditions such as pressure.This can allow a thick article to be kept within the ion sheath even athigh pressures (e.g., 300 to 1000 mTorr). Because the article issupported and carried by the drum, this intimate contact also enablesthe treatment of delicate materials. The intimate contact also ensuresthat plasma fluorination is captured by the article, thereby keeping theelectrode clean. It also allows for effective single-sided treatmentwhen this is desired. However, dual-sided treatment can be achieved bypassing the article through the apparatus twice, with one side beingtreated per pass. A drum electrode also provides a long treatment zone(pix diameter) and provides symmetric distribution of power across theelectrode, which can have operational advantages. The drum may be cooledor heated to control the temperature of the article being treated. Inaddition, linear dimensions in the direction of current flow are madesmall in comparison to the wavelength of the RF radiation, eliminatingthe problem of standing waves.

In other suitable apparatuses, there may be more than one poweredelectrode and more than one grounded electrode. One suitable apparatusfor this invention is a reactor comprising two drum shaped poweredelectrodes within a grounded reaction chamber, which has two to threetimes the surface area of the powered electrodes. The drums can beconfigured so that the article to be treated can travel around and overthe two drums in a manner that allows it to be plasma-treated on bothsides (one side is treated on each drum). The drums may be located in asingle chamber or in separate chambers, or may be in the same chamber,but separated, such that different treatments can occur around eachdrum.

When multiple electrodes are used, they may be powered by a single RFsupply or powered separately. When a single supply is used, the power issometimes distributed unequally between the electrodes. This may becorrected by using a different power supply for each electrode withoscillator circuits linked to a master power supply through a phaseangle adjuster. Thus any power coupling between the electrodes throughthe plasma may be fine-tuned by adjusting the phase angle between thevoltage waveforms of the master and slave power supplies. Flexibility inpower coupling and adjustment between the different electrodes may beachieved by this approach.

In some embodiments, it is desirable to have the grounded electrodewithin about 25 mm of the powered electrode on which an article to betreated is located. Having a grounded electrode close to a poweredelectrode was found to be advantageous. It resulted in articles withhigh levels of fluorination and oil repellency. It was further foundthat, while the proximity of the electrodes provided advantages, it wasnot necessary that the plasma be restricted to the area between theelectrodes. While the plasma glow tended to be brighter between theelectrodes, the plasma filled the entire reaction chamber. In addition,one experiment was carried out in which the grounded electrode wasperforated to more clearly show that the plasma was not confined. Theproperties of the resulting article were as good as those of articlesproduced with an unperforated electrode.

In addition to the capacitive coupling system, the reactor might includeother magnetic or electric means such as induction coils, gridelectrodes, etc.

Methods of Plasma Fluorination

Other aspects of the invention are further directed to methods ofplasma-treating articles. The methods are carried out in a suitablecapacitively coupled reactor system such as those described above.

In different embodiments of methods of the present invention, a groundedand a powered electrode are spaced apart by about 25 mm or less, about16 mm or less, or about 13 mm or less. A low chamber pressure may beused and can be beneficial in some embodiments because the lowerpressure normally allows bigger ion sheaths to form. An article to betreated may be placed on the powered electrode (preferably), thegrounded electrode, or may be suspended between the electrodes. Plasmafluorination of discrete planar articles can be achieved, for example,by suspending an article between the electrodes, preferably abouthalfway between the electrodes. In this embodiment, the article may be,but does not need to be, within an ion sheath. If the article is outsideof an ion sheath, e.g., by being suspended, a treatment time of over twominutes may be required to deposit a fluorinated layer with good oilrepellency properties. However, reducing the space between theelectrodes, e.g., to about 16 mm or about 13 mm, can decrease thenecessary treatment time. Total treatment times of less than two minutescan be achieved if the article is within an ion sheath.

The article to be treated optionally may be pre-cleaned by methods knownto the art to remove contaminants that may interfere with the plasmafluorination. A useful pre-cleaning method is exposure to an oxygenplasma. For this pre-cleaning, pressures in the reactor are maintainedbetween 1.3 Pa (10 mTorr) and 27 Pa (200 mTorr). Plasma is generatedwith RF power levels of between 500 W and 3000 W. Other gases may beused for pre-cleaning such as, for example, argon, air, nitrogen,hydrogen or ammonia, or mixtures thereof.

Prior to the plasma fluorination process, the chamber is evacuated tothe extent necessary to remove air and any impurities. This may beaccomplished by vacuum pumps at a pumping stack connected to thechamber. Inert gases (such as argon) may be admitted into the chamber toalter pressure. Once the chamber is evacuated, a source gas containingfluorine is admitted into the chamber via an inlet tube. The source gasis introduced into the chamber at a desired flow rate, which depends onthe size of the reactor, the surface area of the electrodes, and theporosity of the articles to be treated. Such flow rates must besufficient to establish a suitable pressure at which to carry out plasmafluorination, typically 0.13 Pa to 130 Pa (0.001 Torr to 1.0 Torr). Fora cylindrical reactor that has an inner diameter of approximately 55 cmand a height of approximately 20 cm, the flow rates are typically fromabout 50 to about 500 standard cubic centimeters per minute (scem). Atthe pressures and temperatures of the plasma fluorination (typically0.13 to 133 Pa (0.001 to 1.0 Torr) (all pressures stated herein areabsolute pressures) and less than 50° C.), the source gases remain intheir vapor form.

Upon application of an RF electric field to a powered electrode, aplasma is established. In an RF-generated plasma, energy is coupled intothe plasma through electrons. The plasma acts as the charge carrierbetween the electrodes. The plasma can fill the entire reaction chamberand is typically visible as a colored cloud.

The plasma also forms an ion sheath proximate at least one electrode. Inan asymmetric electrode configuration, higher self-bias voltage occursacross the smaller electrode. This bias is generally in the range of 100to 2000 volts. This biasing causes ions within the plasma to acceleratetoward the electrode thereby forming an ion sheath. The ion sheathappears as a darker area adjacent to the electrode. Within the ionsheath accelerating ions bombard species being deposited from the plasmaonto, and into, the porous article.

The depth of the ion sheath normally ranges from approximately 1 mm (orless) to 50 mm and depends on factors such as the type and concentrationof gas used, pressure in the chamber, the spacing between theelectrodes, and relative size of the electrodes. For example, reducedpressures will increase the size of the ion sheaths. When the electrodesare different sizes, a larger (i.e., stronger) ion sheath will formadjacent to the smaller electrode. Generally, the larger the differencein electrode size, the larger the difference in the size of the ionsheaths. Also, increasing the voltage across the ion sheath willincrease ion bombardment energy.

The article to be treated is placed on or near at least one electrode inthe reaction chamber. In the case of an elongated article, the articleoptionally may be pulled through the vacuum chamber continuously.Contact with an electrode does not need to be maintained. The fluorinespecies within the plasma react on the article's surface and interior. Asuitable plasma could contain fluorine and one or more of oxygen,carbon, sulfur, and hydrogen in various combinations and ratios. Thedegree of fluorination of the final article may be controlled by anumber of factors, for example, the components of the plasma, the lengthof treatment, and the partial pressure of the plasma components. Theplasma fluorination results in species in the plasma becoming randomlyattached to the article surface (including interior surfaces) viacovalent bonds. The deposited fluorine composition may constitute a fulllayer over the entire exposed article surface (including interiorsurfaces), may be more sparsely distributed on the article, or may bedeposited as a pattern through a shadow mask.

Sources of fluorine include compounds such as carbon tetrafluoride(CF₄), sulfur hexafluoride (SF₆), C₂F₆, C₃F₈, and isomeric forms ofC₄F₁₀ and C₅F₁₂, as well as hexafluoropropylene (HFP) trimer (a mixtureof perfluoro 2,3,5 trimethyl 3-hexene; perfluoro 2,3,5-trimethyl2-hexene; and perfluoro 2,4,5-trimethyl 2-hexene, available from 3MCompany).

Other plasma fluorinations might include deposition of amorphous filmsof containing fluorine such as aluminum fluoride, copper fluoride,fluorinated silicon nitride, silicon oxyfluorides, etc. Furthermore,these might include the attachment of additional functional groups.

For treatments with carbon- or carbon-and-hydrogen-rich plasmafluorinations, hydrocarbons are particularly preferred as sources.Suitable hydrocarbon sources include acetylene, methane, butadiene,benzene, methylcyclopentadiene, pentadiene, styrene, naphthalene, andazulene. Mixtures of these hydrocarbons may also be used. Another sourceof hydrogen is molecular hydrogen (H₂). Sources of oxygen include oxygengas (O₂), hydrogen peroxide (H₂O₂), water (H₂O), nitrous oxide (N₂O),and ozone (O₃).

When treatment comprises deposition of a film, it typically occurs atrates ranging from about 1 to 100 nm/second (about 10 to 1000 Angstromper second (A/sec)), depending on conditions including pressure, power,concentration of gas, types of gases, relative size of electrodes, etc.In general, deposition rates increase with increasing power, pressure,and concentration of gas, but the rates will approach an upper limit.

The articles also may be treated in a manner to provide differentdegrees of fluorination in different areas of the article. This can beachieved, for example, by using contact masks to selectively exposeportions of the porous article to the plasma fluorination. The mask maybe attached to the article or may be a separate web that moves with thearticle. By this method, it is possible to obtain fluorinated areas onan article. The fluorinated areas may be in any shape that can beachieved using a shadow mask, e.g., circles, stripes, etc.

Articles having fluorination gradients may also be produced. This can beachieved by exposing different areas of an article to the plasmafluorination treatment for different lengths of time.

In the foregoing description, certain terms have been used for brevity,clarity, and understanding. No unnecessary limitations are to be impliedtherefrom beyond the requirement of the prior art because such terms areused for descriptive purposes and are intended to be broadly construed.Moreover, the description and illustration of the invention is by way ofexample, and the scope of the invention is not limited to the exactdetails shown or described.

EXAMPLES

This invention may be illustrated by way of the following examplesincluding the described test methods used to evaluate and characterizethe plasma fluorinated films produced in the examples.

Plasma Reactor

A parallel-plate capacitively coupled plasma reactor (commerciallyavailable as Model 2480 from PlasmaTherm of St. Petersburg, Fla.),typically used for reactive ion etching, was used to carry out plasmatreatments. The reactor had a chamber that was cylindrical in shape withan internal diameter of 762 mm (30 inches) and height of 150 mm (6inches) and a circular powered electrode having a diameter of 686 mm (27inches) mounted inside the chamber. The powered electrode was attachedto a matching network and a 3 kW RF power supply that was operated at afrequency of 13.56 MHz. The chamber was vacuum pumped with a Rootsblower backed by a mechanical pump. Unless otherwise stated, the basepressure in the chamber was about 1.3 Pa (10 mTorr) or less. Processgases were metered into the chamber either through mass flow controllersor a needle valve. Pressure was controlled independently from flowrateby a butterfly valve. Unless otherwise stated, all the plasma treatmentswere done with the sample located on the powered electrode of the plasmareactor. The samples were taped to the electrode or secured with a metalframe.

Hydrocharging

Some samples were hydrocharged before testing. Hydrocharging can enhancefiltration performance of an article by imparting a permanent charge.Hydrocharging, as taught in U.S. Pat. No. 5,496,507, which isincorporated herein by reference, imparts a permanent charge onto amedia to enhance filtration. This method of hydrocharging comprisesimpinging jets of water or a stream of water droplets onto the sample ata pressure sufficient to provide the sample with filtration enhancingelectret charge. Samples were placed on a mesh belt support and moved ata belt speed of approximately 4 inches/second (10.2 cm/sec) throughwater jets generated by a pump-assisted water sprayer operating at awater pressure of 827 kPa (6206 Torr). The water jets were positionedabout 15 cm (6 in) above the belt. Water was simultaneously removed fromthe sample by vacuum. Both sides of the samples were treated.

The sample was then passed two additional times over a vacuum to removeadditional moisture and then allowed to air-dry overnight beforeproceeding with testing.

Test Methods

DOP Penetration and Pressure Drop Test

Dioctyl phthalate (DOP) loading is a direct measurement of theresistance of a filter medium to degradation due to exposure to an oilymist aerosol. The penetration through, and the pressure drop across, asample were monitored during prolonged exposure of the sample to a DOPaerosol under specified conditions. Standard equipment and testprocedures were used for measuring filter performance.

The measurements were made using an automated filter tester (AFT) Model8130 available from TSI Incorporated, St. Paul, Minn. that was set upwith an oil aerosol generator. DOP % Penetration was calculatedautomatically by the AFT instrument.

DOP % Penetration=100(DOP Conc. Downstream/DOP Conc. Upstream), wherethe concentrations upstream and downstream were measured by lightscattering. The DOP aerosol generated by the AFT instrument wasnominally monodisperse with a mass median diameter of 0.3 micrometersand had an upstream concentration of 85 mg/m³ to 110 mg/m³ as measuredusing a gravimetric filter. Measurements were performed with the aerosolneutralizer turned off and a flow rate through the sample of 42.5 litersper minute (L/min), unless otherwise indicated.

Samples were tested in the following manner. Samples were cut andmounted in a sample holder such that an 11.45 cm (4.5 inch) diameterportion of the sample was exposed to the aerosol. The face velocity was6.9 centimeters/second (cm/sec). Each test was continued until theexposure on the sample was exposed to 200 mg DOP. The DOP % Penetrationand corresponding Pressure Drop data were determined by the AFT andtransmitted to an attached computer where the data was stored.

Quality Factor

Quality Factor (Q Factor) is a measurement of filtration performance. Itdepends on the aerosol used, aerosol flow rate, and filter area. TheQuality Factor of a sample was calculated by the following formula:Quality Factor (Q)=−ln[% DOP Penetration/100]/Pressure Dropwhere Q is in inverse mm H₂O units and Pressure Drop is in mm H₂O units.Q Factors were reported for a DOP penetration loading of 200 mg DOP(Q₂₀₀) at a flow rate of 42.5 L/min and a filter diameter of 11.4 cmresulting in a filter area of 103 cm³.

The higher the Q₂₀₀, the better the filtration performance.

Oil Repellency Test

Porous samples were evaluated for oil repellency using 3M Oil RepellencyTest III (February 1994), available from 3M. In this test, samples werechallenged to either penetration or droplet-spread by oil or oilmixtures having varying surface tensions. Oils and oil mixtures weregiven a rating corresponding to the following:

Oil Repellency Surface Tension Rating Number Oil Composition dynes/cm 0* — — 1 KAYDOL mineral oil 31 2 65/35 (vol) mineral oil/n- 28hexadecane 3 n-hexadecane 26.5 4 n-tetradecane 25.5 5 n-dodecane 24 6n-decane 22 7 n-octane 20.5 8 n-heptane 18.5 *fails KAYDOL mineral oilIn running the Oil Repellency Test, a porous sample was placed on aflat, horizontal surface. A small drop of oil composition was gentlyplaced on the sample. If, after ten seconds it was observed that thedrop was visible as a sphere or a hemisphere, the porous sample isdeemed to pass the test. The reported oil repellency rating of thesample corresponds to the highest numbered oil or oil mixture that wasrepelled.

It was desirable to have an oil repellency rating of at least 1,preferably at least 3.

Fluorine Content

A sample size of about 1 to 3 mg was loaded into an Antek 9000F FluorideAnalysis System available from Antek Instruments, Houston, Tex. Theanalysis was based on oxypyrohydrolysis followed by final analysis witha fluoride ion specific electrode (ISE). The carbon-fluorine bond wasoxypyrohydrolyzed at 1050° C. The product hydrogen fluoride (HF) istrapped in a buffer solution. The dissociated fluoride ions weremeasured with fluoride ISE at a controlled temperature. The calibrationcurve was based on standards prepared with FC-143 (C₇F₁₅CO₂NH₄) in therange of 25 ppm fluorine to 1000 ppm fluorine at an injection of from 10to 15 μL.

Example 1

This example illustrates the effect of the combination of an ion sheathand electrode spacing on Quality Factor (Q-Factor).

A blown microfiber porous article was made from propylene (available asEOD97-13 from ATOFINA Petrochemical, Houston, Tex.) that was extruded ata temperature of 350° C. and blown horizontally onto a collector at adistance of about 300 mm (12 in) from the extruder. The resulting porousarticle had an effective fiber diameter 7.5 μm as described in C. N.Davies, “Air Filtration” Academic Press, 1973. It also had a solidity of7.7%, a basis weight of 87.5 g/m², an effective pore diameter of 25 μm,and a thickness of about 1.24 mm (49 mils). Web thickness was measuredaccording to ASTM D1777-64 using a 230 g weight on a 10 cm diameterdisk. In DOP Penetration testing at 42.5 L/min flow of DOP aerosol, thearticle exhibited a pressure drop of 40 Pa (300 mTorr).

The porous article was cut into rectangles of about 15 cm×30 cm used assamples A to R. The samples were treated on the powered electrode in thePlasma Reactor with plasma formed from perfluoropropane (C₃F₈) gasavailable from 3M Company and with various electrode separationdistances and process conditions as shown in Table 1. The reactorchamber was pumped down to a base pressure of less than 1.3 Pa (10mTorr). C₃F₈ was introduced into the chamber at a flow rate of 100 or200 sccm. Chamber pressure and radio frequency (RF) power wereestablished. A bright plasma was seen in the inter-electrode space andan ion sheath, which was darker than the plasma, formed adjacent to thepowered electrode and encompassed the porous article. For each samplethe plasma treatment was continued for one minute. Then the plasma wasextinguished, the gas flow was stopped, the chamber pressure broughtdown to below 1.3 Pa (10 mTorr), and the chamber was vented toatmosphere. The sample was flipped over and the treatment was repeatedon the other side.

Samples were hydrocharged and measured for DOP penetration. The DOPPenetration Test was run as described in the Test Method section aboveexcept the flow rate was 85 L/min and the neutralizer was on. QualityFactors, Q₂₀₀, are reported in Table 1.

TABLE 1 Q₂₀₀ Spacing Power Pressure Flow (at 85 Sample (mm) (W) (Pa)(sccm) L/min) 1-A 152 1500 37 100 0.398 1-B 152 1000 67 200 0.086 1-C152 2000 67 200 0.120 1-D 152 1000 13 100 0.441 1-E 152 2000 13 1000.335 1-F 152 1500 37 100 0.309 1-G 76 1500 37 100 0.358 1-H 76 1000 67200 0.094 1-I 76 2000 67 200 0.124 1-J 76 1000 13 100 0.445 1-K 76 200013 100 0.422 1-L 76 1500 40 100 0.428 1-M 25 1500 37 100 0.574 1-N 251000 67 200 0.376 1-O 25 2000 67 200 0.556 1-P 25 1000 13 100 0.582 1-Q25 2000 13 100 * 1-R 25 1500 40 100 0.570 * This condition did not runwith a stable plasma. The benefit of reducing the electrode spacing wasclearly seen in the Q₂₀₀ values shown above.

Example 2 and Comparative Example 1

This example illustrates the effect of reduced electrode distance onQuality Factor at the standard test conditions (i.e., 42 L/min andneutralizer off).

Example 2 was made as Example 1-D except a different electrode distance,chamber pressure, and standard test conditions were used as describedherein. The electrode spacing was 0.625 in (16 mm) and chamber pressurewas at 6.7 Pa (50 mTorr). The sample was exposed to the plasma for twominutes on each side. The sample was measured for Oil Repellency. TheOil Repellency Rating was 5. The sample was also hydrocharged andmeasured for DOP penetration. Q₂₀₀ for this sample was 1.53.

Comparative Example 1 was made as Example 2 (except the electrodespacing was 76 mm). The sample was hydrocharged and measured for DOPpenetration. Q₂₀₀ for this sample was 0.58.

The results show that decreasing the electrode spacing provides improvedQ₂₀₀ qualities.

Example 3 and Comparative Example 2

This example illustrates the effect of plasma fluorination within an ionsheath on the oil-repellency characteristics of a porous article.

Example 3 was made as Example 1-D except a different electrode distance,chamber pressure, and standard test conditions were used as describedherein. The electrode spacing was 0.625 in (16 mm) and chamber pressurewas at 16.6 Pa (125 mTorr). The sample was exposed to the plasma for oneminute on each side.

Comparative Example 2 was made in a manner similar to Example 3 exceptthe porous article was suspended in the plasma between the poweredelectrode and the grounded electrode and about 8 mm from eitherelectrode and thus outside the ion sheath. Because a plasma existed onboth sides of the suspended sample, the sample did not have to beflipped over. Total treatment time was two minutes.

Example 3 and Comparative Example 1 were measured for oil repellency.The Oil Repellency Rating for Example 3 and Comparative Example 1 were 5and 4, respectively. The samples were also hydrocharged and measured forDOP penetration. Quality Factors were determined at different amounts ofDOP penetration. The results are shown in Table 2.

TABLE 2 DOP Quality Factor Penetration Example 3 Comp. Example 2 0 2.591.52 20 2.30 1.22 40 2.10 1.02 60 1.93 0.84 80 1.83 0.73 100 1.72 0.62120 1.61 0.54 140 1.51 0.46 160 1.44 0.40 180 1.37 0.35 200 1.28 0.23

As seen in the above table, the Quality Factor at 200 mg of DOP loadingwas 1.28 for Example 3. In contrast, the quality factor of ComparativeExample 2 was 0.23. The Q Factor results indicate that plasmafluorination of a porous sample within an ion sheath was more efficientthan plasma fluorination outside an ion sheath.

Example 4 and Comparative Example 3

The example illustrates the effect of exposure time and electrodedistance on a porous article treated outside of an ion sheath.

Example 4 was made as Comparative Example 2 except the total treatmenttime for the sample was 4 minutes. The resulting sample had an OilRepellency Rating of 4. The sample was hydrocharged and measured for DOPPenetration. A Q₂₀₀ value of 1.28 was obtained.

Comparative Example 3 was made as Example 4. It was made outside an ionsheath with an electrode spacing of 76 mm and for a total treatment timeof 4 minutes. The sample was hydrocharged and measured for DOPpenetration. A Q₂₀₀ value of 0.48 was obtained.

Example 5

This example illustrates the effect of plasma fluorination on theoil-repellency of a porous membrane having small pores.

Example 5 was made as Example 1-D except the porous article wasdifferent and and electrode spacing and chamber pressure were changed.The porous article was a microporous polyethylene membrane madeaccording to U.S. Pat. No. 4,539,256 Ex 8 except the film was stretchedto 6 times its original length in one direction. The membrane had porediameters of about 0.09 micrometer. The electrode distance was about 16mm (0.625 in) and the chamber pressure was 67 Pa (500 mTorr). The samplewas exposed to the plasma for about one minute on each side. Theresulting treated sample had an Oil Repellency Rating of 4. The OilRepellency Rating of the untreated sample was 0.

Example 6

This example illustrates the effect of short exposure times on theoleophobicity of a porous article.

Example 6 was made as Example 1-D except the electrode distance was 16mm, the chamber pressure was 67 Pa (500 mTorr), the total exposure timeswere less than 60 seconds, and the conditions shown in Table 4 wereused. The repellency rating of the untreated sample was 0.

Both samples were tested for oil repellency and DOP penetration. Resultsare shown in Table 3.

TABLE 3 Total Time Power Pressure Flow Repel. Sample (sec) (W) (Pa)(sccm) Rating Q₂₀₀ 6-A 20 1000 67 100 5 1.17 6-B 10 1000 67 100 4 0.80As shown above, Q₂₀₀ was over 1.1 at treatment time of 20 seconds.

Example 7

This example shows the effect of treatment time and proximity to an ionsheath on treatment effect.

The samples each consisted of a four-layer stack of the polypropyleneblown microfiber webs. Each layer was made from polypropylene (availableas EOD97-13 from ATOFINA Petrochemical) that was extruded at atemperature of 330° C. with a collector distance of about 300 mm (12in). The resulting web had an effective fiber diameter of 7.0 μm,pressure drop of 5.9 Pa (44 mTorr), a solidity of 4.7%, a basis weightof 15 g/m² and thickness of about 340 μm (13.5 mils). Each sample stackwas treated with a C₃F₈ plasma in a manner similar to Example 1 but atvarious exposure times and with an electrode separation distance of 16mm (0.625 in). Two samples were made at each of three different exposuretimes, 20 seconds, 120 seconds, and 240 seconds. For each exposure time,one four-layer sample was positioned on the lower, powered electrode(within an ion sheath) and a second four-layer sample was simultaneouslypositioned approximately midway between the powered and groundedelectrodes (outside an ion sheath), which were 16 mm apart. Both thesamples on the powered electrode and the suspended samples were flippedover midway through the treatment. For all samples, the treatmentconditions were 100 seem C₃F₈, 40 mPa (300 mTorr), and 1000 Wattsapplied RF power.

Each sample was analyzed for fluorine content in each of the fourlayers. Exposure times, sample position during treatment, and resultsare shown in Table 4.

TABLE 4 Fluorine Content in ppm Total time 1^(st) 2^(nd) 3^(rd) 4^(th)Sample (sec) Position Layer Layer Layer Layer 7-A 20 Suspended 45 Under5 Under 5 17 7-B 20 electrode 3828 1249 847 2601 7-C 120 Suspended 70 4140 137 7-D 120 electrode 9148 4732 3834 6872 7-E 240 Suspended 146 86 95147 7-F 240 electrode 10475 5539 4826 7598

As seen in the above table, the concentration of fluorine in each of thefour layers of a sample was substantially more for the samples within anion sheath than for those outside the ion sheath.

Example 8

This example illustrates the effect of a perforated electrode on theplasma treatment.

Example 8 was made as Example 2 except the grounded electrode had holeswith diameters of 4.8 mm (0.188 inches) and center-to-center spacings of6.4 mm (0.250 inches), and the chamber pressure was 67 Pa (500 mTorr). Abright plasma was seen everywhere in the chamber including the regionson the side of the perforated grounded electrode opposite the sidefacing the powered electrode.

Example 8 was tested for oil repellency. The Oil Repellency Rating was5. This shows that a perforated electrode, which allowed the plasma tofill the entire chamber more easily than with a standard electrode, hadno detrimental effect on the properties of the resulting article.

Example 9 and Comparative Examples 4 and 5

This example illustrates the influence of electrode spacing on thefluorination of porous and non-porous substrates at comparablevolumetric power densities.

Samples of Example 9 were made in a manner similar to that of Example1-D except the distance between electrodes was varied, and conditionswere changed as described herein. The fluorination treatment was carriedout for a treatment time of 10 seconds with the C₃F₈ gas flow ratemaintained at 100 sccm and the chamber pressure maintained at 67 Pa(0.500 Torr). Samples A and B were flipped over and additionally treatedon the backside of the article for another 10 seconds for a totalexposure time of 20 seconds. RF power was adjusted to nominally maintainthe same power density per unit volume of space between the twoelectrodes for the different electrode distances. The power density forSample A was 0.171 W/cm³. The power density for Sample B was 0.179W/cm³.

Comparative Examples 4 and 5 were made as in Sample A and B,respectively, except the substrate for the Comparative Examples was a0.18 mm thick polycarbonate non-porous film and the Comparative Exampleswere not flipped over during plasma treatment, so the total exposuretime was only 10 seconds on one side. The oil repellency of theuntreated non-porous films was 0.

Samples were tested for oil repellency. The varied process conditionsand results are shown in Table 5.

TABLE 5 Substrate Distance Time Power Repel. Sample Type (mm) (sec) (W)Rating 9-A porous 16.0 20 1000 5 9-B porous 28.5 20 1900 2 CE-4non-porous 16.0 10 1000 6 CE-5 non-porous 28.5 10 1900 6

As seen in Table 5, the results obtained for the porous substrates weredrastically different depending upon the electrode spacing. The porousarticle made with an electrode spacing of 16 mm withstood a No. 5 fluidin the Oil Repellency Test whereas the porous article made with anelectrode spacing of 28.5 mm withstood only a No. 2 fluid. In contrast,non-porous samples were not affected by the electrode spacing.

Example 10

In order to understand the effect of deposition rate of the fluorocarbonon a porous sample, the treatment conditions used to make Samples 9-Aand 9-B were repeated on Samples 10-A and 10-B, respectively. Thesubstrates for samples 10-A and 10-B were pieces of silicon over which apolystyrene film had been spin-coated. Portions of the substrates weremasked with tape to allow for step-height measurements using a stylusprofilometer available as Alpha-Step 500 from Tencor Instruments,Mountainview, Calif. The samples were not flipped over. Total exposuretime was 120 seconds, chamber pressure was 67 Pa (500 mTorr) and gasflow rate was 100 sccm. Power was varied as described above to maintaincomparable power densities.

Samples were tested for oil repellency. The process conditions anddeposition rate results are shown in Table 6.

TABLE 6 Time Power Distance Flow Deposition Rate Sample (sec) (W) (mm)(sccm) (nm/s) 10-A 120 1000 16 100 2.16 10-B 120 1900 28 100 2.27

The measured deposition rate of 2.16 nanometers/second for sample 10-Awas nominally the same as the rate of 2.27 nanometers/second for sample10-B. Thus the superior repellency performance of Sample 9-A over Sample9-B was not due to a higher deposition rate and thicker film. Thisillustrates that the superior article properties provided by theinvention are not due to depositing thicker fluorinated layers, but aredue to more efficient plasma fluorination of article interiors.

Example 11

This example illustrates the benefit of locating the porous substrate onthe powered electrode for short treatment times.

Samples for Example 11 were made as in Example 1-D except the electrodeseparation distance was 16 mm (0.625 in) and some process conditionswere different as described herein. Sample A was located on the poweredelectrode whereas sample B was located on the grounded electrode. Bothsamples were secured to the electrode with removable Scotch tape on theedges. Fluorination was done at a chamber pressure of 67 Pa(500 mTorr)with a C₃F₈ flow rate of 100 sccm, and RF power maintained at 1000 W.Both the samples were treated for 10 seconds, then flipped over andtreated on the opposite side for another 10 seconds for a totaltreatment time of 20 seconds.

The samples were tested for oil repellency and the results aresummarized in Table 7.

TABLE 7 Electrode Substrate Spacing Time Power Repel. Sample Location(mm) (sec) (W) Rating 11-A Powered 16.0 20 1000 5 Electrode 11-BGrounded 16.0 20 1000 2 Electrode

As seen in the table, the oil repellency rating of the sample located onthe powered electrode was significantly better than the sample locatedon the grounded electrode.

Example 12

This example demonstrates the efficacy of the fluorination process whenthe electrode spacing is less than 12 mm (0.5 in). Stable plasmaoperation is generally not possible with such a small spacing. Byoperating the C₃F₈ plasma at a pressure of 67 Pa (500 mTorr) and powerof 1000 Watts, a surprisingly stable plasma was obtained even when theelectrode spacing was as low as 6.3 mm (0.25 in). Samples for Example 12were made as in Example 1-D except the electrode separation distance was8.6 mm (0.340 in) for sample 12-A and 6.3 mm (0.25 in) for samples 12-Band 12-C. Fluorination was done at a chamber pressure of 67 Pa (500mTorr) with a C₃F₈ flow rate of 100 sccm, and RF power maintained at1000 W. Samples 12-A and 12-B were treated for 10 seconds, then flippedover and treated on the opposite side for another 10 seconds for a totaltreatment time of 20 seconds. Sample 12-C was treated in the same mannerusing the same process conditions but the treatment time was for 5seconds per side, a total treatment time of 10 seconds. The OilRepellency Ratings of these samples are summarized in Table 8.

TABLE 8 Electrode Substrate Spacing Time Power Repel. Sample Location(mm) (sec) (W) Rating 12-A Powered 8.6 20 1000 5 Electrode 12-B Powered6.3 20 1000 5 Electrode 12-C Powered 6.3 10 1000 5 ElectrodeAs can be seen from the data, the Repellency Rating is excellent evenwhen the treatment times are as small as 10 seconds.

Example 13

This example demonstrates the effect of treating a porous article on thegrounded electrode with a small electrode spacing.

Samples of the web described in Example 1 were plasma fluorinated at aC₃F₈ flow rate of 83 sccm, a chamber pressure of 40 Pa (300 mTorr), RFpower maintained at 1000 Watts, and an electrode spacing of 16 mm.Sample 13-A was placed in the ion sheath adjacent to the poweredelectrode while sample 13-B was placed in the ion sheath adjacent to thegrounded electrode. The samples were hydrocharged and tested for DOPpenetration using the standard test method. Q₂₀₀ for Example 13-A was1.24. Q₂₀₀ for Example 13-B was 1.06.

Various modifications and alterations of this invention will becomeapparent to those skilled in the art without departing from the scopeand spirit of this invention and it should be understood that thisinvention is not to be unduly limited to the illustrative embodimentsset forth herein.

1. A method of fluorinating a porous article comprising: providing areaction chamber containing a capacitively-coupled system comprising atleast one grounded electrode and at least one drum electrode powered byan RF source separated from the grounded electrode by less than 25millimeters; generating a fluorine-containing plasma in the chamberthereby causing an ion sheath to form adjacent to the electrodes;placing a porous article in the ion sheath of the powered electrode suchthat the porous article is in contact with the at least one drumelectrode; and allowing reactive species from the plasma to react withthe article surface and interior whereby the article becomesfluorinated; wherein the grounded electrode is substantially parallel tothe surface of the powered electrode and the porous article ispositioned between the electrodes.
 2. The method of claim 1 wherein thearticle has pores that are smaller than the mean free path of anyspecies in the plasma.
 3. The method of claim 1 wherein the porousarticle is substantially continuous.
 4. The method of claim 1 whereinthe-reactive species from the plasma reacts with the article surface andinterior for less than 60 seconds.
 5. The method of claim 1 wherein theporous article is selected from the group consisting of foams, wovenmaterials, nonwoven materials, membranes, frits, porous fibers,textiles, and microporous articles.
 6. The method of claim 1 wherein thearticle has two parallel major surfaces and is plasma treated on onemajor surface by allowing reactive species from the plasma to react withthe article surface and interior, whereby the article becomesfluorinated.
 7. The method of claim 6 wherein the article is furtherplasma treated on its second major surface by allowing reactive speciesfrom the plasma to react with the second major surface and interior,whereby the second major surface of the article becomes fluorinated. 8.The method of claim 1 where the electrodes are separated by about 16millimeters or less.
 9. A method of fluorinating a porous articlecomprising: providing a reaction chamber containing acapacitively-coupled system comprising at least one drum electrodepowered by an RF source and at least one grounded electrode that issubstantially parallel to the surface of the powered drum electrode andseparated from the grounded electrode by about 25 millimeters or less;generating a fluorine-containing plasma in the chamber at a pressure ofabout 40 Pascal or less; placing a porous article between thesubstantially parallel electrodes such that the porous article is incontact with the at least one drum electrode; and allowing reactivespecies from the plasma to react with the article surface and interiorfor a total treatment time of over two minutes whereby the articlebecomes fluorinated.
 10. A method of fluorinating a porous articlecomprising: providing a reaction chamber containing acapacitively-coupled system comprising at least one drum electrodepowered by an RF source operated at a frequency of 13.56 MHz and atleast one grounded electrode that is substantially parallel to thesurface of the powered electrode and separated from the groundedelectrode by about 13 millimeters or less; generating afluorine-containing plasma in the chamber thereby causing an ion sheathto form adjacent to the electrodes; placing a porous article between theelectrodes such that the porous article is in contact with the at leastone drum electrode; and allowing reactive species from the plasma toreact with the article surface and interior whereby the article becomesfluorinated.